The present invention relates to a system for cleaning the inside diameters in a bank of tubes in a heat exchanger system, tube cleaners for use therein and methods for their use.
Heat exchangers of the type contemplated by the present invention comprise condensers utilized in applications such as steam generating power plants. A condenser includes tubes arranged in a tube bundle. Water flowing through the condenser tube bundle picks up heat from condensing steam on a shell side or outside diameters of the tubes. Based on conventional design considerations, a plurality of condensers may be installed in one power plant, each condenser having a large number of tubes. The selection of number of tubes and number of condensers is a function of the design parameters for each system. These design parameters include the amount of water to be pumped through the tubes, the temperature of steam contacting the tubes and various flow rates. According to R. J. Stoker and E. F. Seavey, "The Selection of Large Steam Surface Condensers," Combustion, 1967, in a survey of fifty condenser units in a nominal 600 megawatt plant, a nominal flow through the condensers could be 300,000 gallons per minute. In one example, 300,000 square feet of surface area in the condenser are required for cooling. In a nominal design, this surface is provided using tubes 50 feet long and having an outside diameter of one inch. Since maximum surface area is provided by long, small diameter tubes, it is easy to see that individual condensers having 40,000 tubes in a bundle would not be uncommon.
It will be readily apparent that any buildup of coatings inside these tubes or any other fouling will provide the highly disadvantageous effects of buildup and back pressure, making it more difficult to circulate the required large quantities of water through the tubes, and of creating a thermal insulation on the inner surface of the tubes, tending to defeat the very purpose of the heat exchanger itself. Since large quantities of water flow through the tubes and small buildups have a highly disadvantageous effect, online cleaning is very important in maintaining the efficacy and economy of the system. The continuous removal of deposited scale from the insides of the tubes will prevent pitting or concentration cell attack, a form of galvanic corrosion and will prevent further scale deposition. This will further prevent destruction of the tubes. Advantages of a cleaning program are further discussed in "Condenser Cleaning Improves Economics," Electrical World, Dec. 15, 1969, page 31, and in A. F. Stegelman and R. Renfftlen, "Online Mechanical Cleaning of Heat Exchangers," Hydrocarbon Processing, January 1983, page 95.
An established method of cleaning consists of circulating sponge rubber balls through the heat exchange unit. The balls are forced by pressure to traverse the tubes and each wipe the inside of a tube. While a sponge rubber ball will have only a minor effect on one pass, the balls are commonly maintained in circulation through several hundred or several thousand passes with the objective of cleaning the interior diameter of the tubes. Apparatus must be provided for collecting the sponge rubber balls after they exit from the tubes, conducting them through a recirculation path and reinjecting them into a liquid stream for reintroduction into the tubes. Therefore, the art has developed various forms of cleaners and systems for circulating them repeatedly through tube bundles. The present invention seeks to provide advantages not found in the prior art and elimination of disadvantages as are included in the prior art further discussed here.
Exemplary of many prior art systems is U.S. Pat. No. 2,801,824, issued on Aug. 6, 1957, to J. Taprogge. In the apparatus disclosed therein, condenser tubes are automatically cleaned by cleaning elements comprising "rubbing bodies" which are carried along tube walls by a liquid medium such as the cooling water. The rubbing bodies are moved through the heat exchanger in continuous circulation and intercepted from the outlet of the spent medium by a suitable device and returned again into the fresh liquid medium supplied to and flowing through the heat exchanger. The suitable device comprises a funnel shaped strainer in this apparatus. Another such system is disclosed in U.S. Pat. No. 4,351,387, issued on Sept. 28, 1982, to L. Milia. In the disclosure therein, the suitable device is a sieve means which may be formed as a "V" to intercept the outlet flow from the condenser when the system operates in a mode for recirculating the cleaning means. Alternatively, each leg of the "V" may be rotated so that the two legs are parallel and flow without intercepting the outlet from a condenser when a cleaning operation is not being performed. Many other arrangements are provided in which a means intercepting the entire outlet from the condenser is positioned to intercept the flow and in which the intercepting means may be "feathered" to permit the flow to pass without being screened.
The screens themselves create a pressure drop. They may be subject to clogging, depending on the nature of contaminants found in the cooling water to exacerbate the magnitude of the pressure drop. The above-described construction requires the use of strong, durable screens to withstand the full flow issuing from the condenser. The price of a cleaning system is considerable. In an exemplary 800 megawatt steam generating station built around 1975, a nominal price for the equipment attributable to the system for recirculating cleaning elements, separate and apart from the condensers themselves, was one million dollars. It is desirable to provide a system in which simpler construction is possible and in which it is not necessary to intercept the entire flow issuing from the condenser station.
As to the cleaning balls themselves, many forms have been provided commonly utilizing sponge rubber of a slightly larger dimension than the inner diameter of the tube to be cleaned. Over a large number of recirculating passes through the tubes, the sponge rubber balls tend to remove undesired buildups in the interior of heat exchanger tubes. After separation from the outlet stream from the condenser, the cleaning elements are recirculated. A means for separating worn out balls which are decreased in size prior to delivery to the inlet stream to the condenser may be provided.
The cleaning elements are normally unevenly distributed through the tubes within the banks in a condenser since the cleaning elements are heavier than water when recirculated and returned for introduction into tubes in a condenser. As stated above, there can be up to 40,000 tubes in a bank. Therefore, some tubes in a bundle will be 20 feet higher than others. Turbulence normally encountered in an inlet manifold at the entrances to the tubes will not overcome the effects of gravity on the heavier-than-water cleaning elements in terms of providing for uniform vertical distribution of cleaning elements. Tubes which are vertically above other tubes will therefore tend to have fewer cleaning elements circulating therethrough with uneven cleaning resulting. Consequences can be significant. Upper tubes may not be cleaned effectively and eventually must be mechanically and/or chemically cleaned. Acid cleaning is a known form of cleaning. There is the attendant expense of the cleaning operation and significant "downtime" for the condenser, which often results in the shutting down of the power plant itself.
In circulating, the cleaning elements must strike against the screens, separating sieves or the like, to be removed from the outlet flow and recirculated. The necessary action of this separation reduces the useful life of the cleaning elements. It is desirable to have a system in which separation does not require the rolling of cleaning elements against a screen.
The geometry of the cleaning elements can provide for difficulty and expense in their construction. It is desired to provide for the option of simplifying construction.
Prior art refers to means used for cleaning inner diameters by many different terms, e.g. plug, pig, ball, etc. Such names will be referred to herein as tube cleaners.